Preliminary work has identified a proteolipid prepared from renal brush border membranes that binds inorganic phosphate with high affinity and specificity. These and other features suggest that this may be the phosphate transporting protein involved in renal reabsorption of phosphate. This protein, then, is a condidate to be the first carrier or porter to be identified in epithelial transport. Identification of a carrier for a molecule (phosphate) that is co-transported with sodium and that is under potential multihormaonal control should lead to extensive new information about membrane transport processes and their regulation in epithelia. The aims of the present project include purification of this hydrophobic phosphate binding protein, characterization of its binding parameters and reconstitution of the transport capacity in vesicles. In addition, we will examine the role of this protein with respect to agents (parathyroid hormone, vitamin D) and conditions (dietary phosphate deprivation) known to alter phosphate transport.